I. Field of the Invention
The present invention relates generally to medical implants and, more particularly, to implantable prostheses and materials used for same. The invention also relates to a process for making such materials.
II. Related Art
Medically implantable prostheses, exemplified by breast implants, are well known in the art. Such implants generally comprise a formed body presenting a nonreactive, biocompatible outer surface to surrounding tissue following implantation. Fluid-filled medical implants generally comprise a viscous fluid contained within an elastomeric shell. It has been observed that fluid-filled medical implants may leak or rupture following implantation and require explanation. The escaping fluid filler material may be contained within a periprosthetic capsule that forms around the prostheses after implantation, or it may be released into the body. It would present a desirable advantage to provide a filler for an implantable soft tissue prosthesis wherein the filler itself is substantially cohesive to facilitate removal of the filler from the body in the event of a rupture. It would present an additional advantage were this material composition nontoxic and preferably bioabsorbable.
Filling materials disclosed in previous patents relating to breast implants containing the polymer polyvinylpyrrolidone (PVP) have had some drawbacks that have been demonstrated and reported in the cosmetic plastic surgery literature. These implants generally are constructed with a silicone membrane shell. These drawbacks relate to the osmotic pressure created within the silicone shell membrane of the breast implant by the PVP solutions that have been previously utilized as filling material in clinical settings. The drawbacks also relate to control of the viscosity, cohesiveness, and elasticity of the PVP mixture used for filling breast implants.
Cross-linked PVP has a history of patented processes for the preparation of cross-linked PVP products, now commercially available from two major corporations: ISP and BASF. Three such patents are U.S. Pat. Nos. 2,938,017, 3,759,880, and 3,933,766. In the previous literature describing the process for obtaining cross-linked PVP, the temperature at which the cross-linking of PVP occurred has been required to be 100° C. or higher. Known processes for cross-linking PVP have required compounds or conditions which make them difficult to control. The rapid rate of cross-linking PVP in the aforementioned patents prohibits precise control of the cross-linked PVP products. Prior processes including that described in U.S. Pat. No. 3,933,766 call for the use of a cross-linking compound such as a cyclic acid amide or alkoxides in high pH environments (10-12 pH) or special commercial chemical “cross-linkers” at temperatures of 150° C. and pressures of 100 mm Hg.
Tacky, hydrophilic gel dressings have been disclosed using poly(N-vinyl lactam)-urethane gels in which the poly (N-vinyl lactam) may be polyvinylpyrrolidone (PVP) in U.S. Pat. Nos. 5,156,601 and 5,258,421. A skin adhesive hydrogel formed by mixing high molecular weight PVP having ring opened pyrrolidone groups and a multi-functional amine-containing polymer is disclosed in U.S. Pat. No. 5,306,504.
A further patent to Hayes et al, U.S. Pat. No. 5,997,574, discloses a medical implant which includes a shell and a filler material in which the filler material requires a combination of materials, namely, both a rheological agent and an osmotic control agent that is different from the rheological agent. In particular, Hayes et al teaches the use of polyvinylpyrrolidone (PVP) as an osmotic control agent in combination with a gum, preferably guar gum, as the rheological control agent. The combination is considered essential in order to provide a filler material having the desired properties for a breast implant, for example. The filler material of Hayes et al, further, is a one-phase, water-soluble mixture.
A long-standing need in the art for an improved formulation of PVP mixture for a filling material in breast implants has been recognized by the inventors and a new formula has been compounded and proposed. Accordingly, one aspect of the present invention relates to an improvement in cohesiveness, osmolarity, elasticity, and the viscosity of cross-linked PVP mixtures and mixtures of cross-linked PVP derivatives so that they can be controlled to be more favorable as filling material for breast implants and other uses. The proposed material composition is a viscous, highly elastic and cohesive mass comprised primarily in one embodiment of a lattice of water-insoluble, heat-treated, cross-linked PVP and water.